Add a "last updated" timestamp to the leaderboard

[Muskankr]

Since this is aggregated data (likely not real-time), show users when it was last refreshed so they understand data freshness.

Acceptance criteria:

Timestamp displayed near the page heading or filter tabs
Backend tracks and returns the last aggregation/refresh time
Reasonable relative formatting (e.g. "Updated 3 hours ago")

[Ayush-0918]

I would like to work on this issue. Please assign to me.

[Ayush-0918]

Hi! I started working on this issue. While implementing it, I found that it depends on the Skills Leaderboard introduced in #383, but that code isn’t currently available on the main branch. Because of that, the timestamp changes can’t be applied cleanly against main. Could you please confirm whether I should:

• wait until the leaderboard feature is merged into main, or
• target a different base branch for this PR?

[Muskankr]

@Ayush-0918 , wait for now

[Ayush-0918]

@Muskankr Okayyyyy